Well you could use an API , have a look on Aloe and have endpoints handling that , I guess this is the most flexible way, other way is to share somehow that folder via ftp or ssh and get the folder but taking in consideration that it will reside on customers side then I guess API for you, or last resort you could use WEBDav I guess or a cloud server to handle your files.

Well you could use an API , have a look on Aloe and have endpoints handling that , I guess this is the most flexible way, other way is to share somehow that folder via ftp or ssh and get the folder but taking in consideration that it will reside on customers side then I guess API for you, or last resort you could use WEBDav I guess or a cloud server to handle your files.